Future Batteries, by the Numbers

After a decade of intense focus on lithium-ion, material scientists are beginning to look to new battery chemistries for future electric cars.

The new chemistries could require a long wait -- 10 or 20 years in some cases -- but they might change the future of electric transportation. "We're getting to the end of the road as far as being able to extract more energy out of lithium-ion," John Kopera, vice president of commercial operations for Sion Power, a maker of lithium-sulfur batteries, told us. "But we can see a time when batteries could have higher energy, weigh less than half as much, and still be less costly than lithium-ion."

Sion Power isn't alone in believing better battery chemistries are on the horizon. The Advanced Research Projects Agency (ARPA-e) has tabbed a variety of chemistries for government-sponsored development through its Batteries for Electrical Energy Storage in Transportation (BEEST) program, including lithium-sulfur, lithium-air, zinc-air, and magnesium. Other materials scientists are targeting aluminum-ion and even lead-acid as possible EV solutions.

Using lead-acid chemistry with enhanced power density, Energy Power Systems hopes to replace the nickel-metal hydride batteries in hybrid cars with batteries that cost less than half as much.

The road to EV battery nirvana is fraught with difficulties. It has taken more than a century to reach today's state. Moreover, the EV battery industry has been notorious for overpromising and underdelivering on its technology. "People who develop batteries often don't appreciate all the problems that are involved," Elton Cairns, a professor of chemical engineering at the University of California-Berkeley, who designed fuel cells for the Gemini space programs, told us. "Each new idea looks really great until you get farther along and discover all the problems."

Still, battery chemistries such as nickel-metal hydride and lithium-ion have provided great gains for the auto industry over the last 20 years. Experts say more are on the way, especially if automakers and consumers appreciate the fact that battery development takes painstaking work and patience.

Lithium evolution
The wait for lithium-ion was a long one. John Goodenough, a professor of mechanical engineering and material science at the University of Texas, began exploring lithium-iron-phosphate chemistries in the late 1970s. The technology hit the automotive radar in the early 1990s, and it took two more decades for it to reach its current level.

Some experts say lithium-ion may be reaching its limits. Though development continues on lithium-ion batteries that are said to offer specific energies of more than 300Wh/kg, many materials scientists say lithium-ion won't exceed its current level of 150-200Wh/kg. "Mature battery technologies typically reach about 40 percent of their theoretical energy," Kopera said. "Lithium-ion is already at 40 percent."

The reason is simple. In practice, battery designers must add dead-weight components (such as electrolytes, terminals, and housings) that boost the battery's mass and thereby reduce its specific energy. Scientists say lithium-ion is reaching the point where mass reduction is getting more difficult.

Chuck, that's a good point. I like to let my car run down to empty, if I am in an area with lot's of gas stations, becuase it will get slightly better mileage toward the end of the tank. Another thing to note is the difference weight of the engines. I don't know what the Tesla S engine weighs, but I was told that the Tesla Roadster engine weighs only 70 lbs. Add to that the fact that there is no transmission, and the S engine has to weigh a lot less than the 550i engine.

One thing this makes clear is that when comparing two very different technology systems one has to consider more than the core part of the system. In this case the motors are vastly different and just comparing them would leave you to believe that the electric car should be lighter. Add in the energy storage system, and you come up with a very different story.

You're right, Naperlou. You are essentially saying that the specific energy of gasoline is far higher than that of a lithium-ion battery. And, yes, that's a drawback. It's also a drawback when the battery's charge is depleted. The 900 lb battery still weighs the same. It still has to carry its own dead weight, whereas the weight of the gasoline in your example has gone from 73 lbs to zero when the gas tank is depleted.

Cap'n, I read this article with interest. While doing so I looked up a number of things. What I found was suprising. One comment you made was the search for a $5K battery pack that got 300 miles of range. Tha is a long way off, as you mention.

With a gasoline engine at 23 mpg (see below), the 300 mile range translates to 13.04 gallons. The weight of the gasoline is 72.88 lbs. Tesla claims that range for their Tesla S with the 85 kW-h battery. The weight seems to be about what the roadster's is at a minimum. That would be 900 lbs. The ICE gets 0.26 lbs./mile, while the Tesla S has a rating of 3 lbs./mile. So, the first thing that needs to be addressed is the weight of the battery. The Tesla S battery has density of 94.44 W-h/lb. This is at the high end of the range you quote for current batteries (note the unit difference).

What is even more interesting is that the curb weight for a Tesla S is 4,647.3 lbs. That's a lot. The roughly equivalent BMW 5 series sedan (which Tesla is targeting with the S) is the 550i sedan. The power output is similar, The BMW weighs 4,365 lbs. I find this very interesting for both given all the articles and discussion on weight saving, etc. that the car manufacturers are supposedly working on.

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